Complementary metal oxide semiconductor (CMOS) integrated-circuit image sensors that are highly tolerant of high-energy radiation ("radiation hard") are undergoing development. These devices are sensitive to visible light, yet are insensitive to diverse types of highly energetic radiation, including protons at kinetic energies >1 MeV, neutrons, heavy ions, high-energy g rays, x rays, and electrons. They can continue to function substantially as designed, even after exposure to radiation doses in excess of 10 megarad (Si).

Figure 1. In a Conventional CMOS Pixel Photodiode, interaction between the LOCOS region and the depletion region is the primary source of susceptibility to radiation-induced generation of dark current.
The integrated circuits now used predominantly for visible-light image sensing are charge-coupled devices (CCDs), which are notoriously radiation-soft due to the need for repeated and accurate charge transfer. CMOS image sensors do not require charge transfer and hence are more suited for operation in radiation-rich environment. The main mechanism of degradation of a CMOS imager under radiation is the increase in dark current with total dose. The present development efforts are directed toward minimizing radiation-induced increases in dark current in CMOS image sensors.

Figure 1 is a simplified depiction of a cross section of a typical photodiode used as the photodetector in one pixel of a conventional CMOS image sensor. The photodiode is formed between the n+ implant and the p substrate. The field oxide provides electrical isolation between pixels. The thickness of the field oxide is chosen to ensure that the underlying silicon cannot be inverted at any bias applied on the oxide. The transition between the field oxide and the thin oxide that covers the n+ implant is denoted either the bird's beak (because of its appearance in cross section) or the LOCOS region [because it is fabricated by the local oxidation of silicon (LOCOS) process (a process of masked oxidation near the surface of the silicon substrate)].

The LOCOS region is under elevated mechanical stress, and the densities of dangling molecular bonds and interface traps in this region are greater than that at the interface between the thin oxide and the n+ Si. Because the n+ region is in electrical equilibrium, the interface traps above the n+ region do not substantially participate in the generation of dark current. However, the LOCOS region intersects the depletion region, which is in electrical non-equilibrium.

Figure 2. Radiation-Hard CMOS Pixel Photodiodes differ from conventional ones by incorporating features that suppress the deleterious effects of interaction between the LOCOS and depletion regions.
High-energy radiation creates additional dark-current-generating centers at interfaces between silicon and silicon dioxide. In the presence of thermal excitation, these centers provide excess electrons that collectively manifest themselves as increased dark current. The generation centers are usually located in the LOCOS region because of the concentrations of stress and electric field there. Hence, a major part of the effort to reduce radiation-induced dark current has been focused on the LOCOS region.

Another part of the effort involves a radiation-hard CMOS design with superior immunity against latch-up, minimal shift of threshold voltage per unit dose of ionizing radiation, and increased stability against electric-field inversion. A third major part of the effort involves a circuit design that provides for source-follower bias field-effect transistors (FETs) arranged in a current-mirror configuration. Current-mirror-based biasing schemes are intended to provide constant bias currents, irrespective of operating temperatures and radiation doses.

Figure 2 depicts cross sections of the photodiodes in three different pixel designs of radiation-tolerant CMOS image sensors that have been investigated thus far. In the surround-gate pixel, the n+ implant is separated from the LOCOS region by a polycrystalline silicon (polysilicon) gate that surrounds the implant. A sufficiently negative DC potential on the polysilicon gate (relative to the p-silicon substrate) would prevent formation of a depletion region adjacent to the LOCOS region. The negative potential would hold the region around the n+ implant in accumulation, thereby passivating the interface states in the LOCOS region and thereby, further, suppressing radiation-induced dark current.

In the implant-setback pixel, the edge of the n+ implant is deliberately set back from the LOCOS region to prevent interaction between the LOCOS region and the depletion region around the implant. Holding the LOCOS region in electrical equilibrium is expected to reduce the generation of dark current. A threshold-adjust implant is expected to prevent the formation of a depletion region at the surface and to provide additional radiation hardness.

In the silicide-bounded pixel, a thin silicide film is formed over an annular region around the edge of the n+ silicon implant. The formation of the silicide film is expected to reduce the stress in the LOCOS region, thereby reducing the probability of formation and/or activation of dark-current-generating centers in the LOCOS region.

Tests of the three designs have led to two preliminary conclusions. The first conclusion is that silicide-bound-pixel design effects a significant reduction of both pre-irradiation andradiation-induced dark current. The second conclusion is that the silicide-bound-pixel and implant-setback-pixel designs are the most promising for implementing radiation-hard CMOS image sensors.

This work was done by Bedabrata Pain, Bruce Hancock, and Thomas Cunningham of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Electronics & Computers category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Intellectual Property group

JPL

Mail Stop 202-233

4800 Oak Grove Drive

Pasadena, CA 91109

(818) 354-2240

Refer to NPO-30185, volume and number of this NASA Tech Briefs issue, and the page number.


This Brief includes a Technical Support Package (TSP).
Radiation-Tolerant CMOS Image Sensors

(reference NPO-30185) is currently available for download from the TSP library.

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This article first appeared in the December, 2001 issue of NASA Tech Briefs Magazine.

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